<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="text" id="i2">
<input type="button" value="加" onclick="f1()">
<input type="button" value="减" onclick="f2()">
<input type="button" value="乘" onclick="f3()">
<input type="button" value="除" onclick="f4()">
<h1>结果:<span></span></h1>
<script>
    let i1 = document.querySelector("#i1");
    let i2 = document.querySelector("#i2");
    let s = document.querySelector("span");
    function f1() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误!";
            return;
        }
        s.innerText = i1.value*1+i2.value*1;
    }
    function f2() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误!";
            return;
        }
        s.innerText = i1.value-i2.value;
    }
    function f3() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误!";
            return;
        }
        s.innerText = i1.value*i2.value;
    }
    function f4() {
        if (isNaN(i1.value)||isNaN(i2.value)){
            s.innerText="输入错误!";
            return;
        }
        s.innerText = i1.value/i2.value;
    }


</script>
</body>
</html>